The non-linear differential equations describing the behavior of a population of molecules, or of cells, or of individuals, are studied in the context of systems theory. Necessary and sufficient conditions for the existence of enclosures are sought, and their biological importance investigated. BIBLIOGRAPHIC REFERENCES: Rescigno, A.: The Struggle for Life -IV. Two Predators Sharing a Prey. Bull. Math. Biol. 39: 179-185, 1977. Rescigno, A.: The Struggle for Life - V. One Species Living in a Limited Environment. Bull. Math. Biol. 39: 1977 (In press).